Origin of the concept of «Oneness» in (Zen-) Buddhism

I know that Zen-Buddhism knows a form of practise to achieve full harmony between the individual, his/her action and the animate or inanimate environment, often translated as «Oneness». - Does anybody know the roots of this practise? - Does it go back to Gautama Buddha or to an other known Buddhist teacher? (Background: Around the year 800 CE (1300-1400 BE), Sufi Islam, and, shortly later, Christian and Jewish Mysticism have developed a concept of «Oneness» to reach by Dikr or meditation, meaning to reach a full compliance with the God; God meaning the Whole that is in relation to us. This concept does not have direct roots in either the Quran or the Bible. Although it might have been an independent parallel to Buddhism, I rather suppose that this concept is inspired by the Buddhist religion, above all because at the same time, even the concept of rebirth has been taken over by Jewish Kabbala and Alewite Islam although it is incompatible with traditional concepts of the three religions (which, in contrast to the aforementioned Sufi/Mystic practises have not entered into the main stream of either religion). It is part of the question whether we Muslim could or should recognise Gautama Buddha or possibly a Buddhist after him as a Prophet or as a valuable source of wisdom of Islam; I will have to discuss this question in Islam rather than here.)